A typical handset for, e.g., a conventional or cellular telephone, includes a handgrip connecting a receiver or earpiece at one end to a transmitter microphone situated at the other end. Although the earpiece is typically held at close proximity to the user's ear, the user's ear often receives enough of the ambient noise to reduce the intelligibility of the incoming voice signal. For this reason, the utility of a conventional handset often suffers in noisy environments such as airports, city streets, and the interiors of moving vehicles.
One expedient that has been applied to this problem is a volume control for the handset. In at least some instances, the volume control can be used to render incoming speech more intelligible by increasing the volume of the incoming speech signal relative to the ambient noise level. However, simply turning up the volume is not always a perfect, or even an adequate, solution to the problem. For example, the incoming voice signal itself may be so corrupted with noise that increasing the volume provides little or no advantage. As another example, the ambient noise may be so loud, especially if it is concentrated in a relatively narrow range of frequencies within the band most critical for speech intelligibility, that the volume cannot be turned up high enough to compete with it without inflicting pain on the user.
One method for reducing the corruption of a received signal by ambient noise is to apply active noise cancellation. A typical circuit for active noise cancellation creates an inverted version of the undesired noise signal. This inverted signal is then added to the incoming signal stream.
Although it is not typical, in the prior art, to employ active noise cancellation in telephone handsets, this technique has been used in headsets such as those used by the operators of aircraft. In a noise-canceling headset, the inverted signal is applied to the earpiece, where it subtractively interferes with the undesired noise signal within the user's ear. Headsets of this kind are described, for example, in U.S. Pat. No. 5,343,523, entitled "Telephone Headset Structure for Reducing Ambient Noise", commonly assigned herewith.
Numerous difficulties have beset earlier efforts to apply this noise-canceling to handsets, rather than headsets. In particular, a headset exerts firm control over the positioning of the earpiece relative to the user's ear, and it also effects a seal between the earpiece-ear system and the outside world. By contrast, a handset typically provides little or no such seal, and by virtue of its manipulation by a user, it experiences widely variable positioning of the earpiece relative to the user's ear. For these reasons, it is difficult in a handset to match the phase and amplitude of the inverted signal to the actual noise. Without a substantial amount of such matching, there will generally lack a useful degree of noise cancellation.
One approach to this problem is described in U.S. Pat. No. 5,491,747, entitled "Noise-Cancelling Telephone Handset," commonly assigned herewith. In the handset described there, the receiver loudspeaker is mounted in a receiver cap conformed with a domed, apertured element. The dome fits into a user's outer ear to aid in positioning the handset. Although such a handset is useful for achieving better noise cancellation, it may meet with resistance in some commercial markets because of practical constraints on how it can be held during use, and because some customers might object to its external appearance, which includes a protrusion on the outer face of the earpiece for containing the monitoring microphone.
Thus, until now, there has lacked a communication handset that can provide effective active noise cancellation, and thus improve the intelligibility of incoming voice signals, even when subjected to individual and user-to-user variations in the manner in which it is held during use.